Cisco CCNA Study Guide - Router Alley

CCNA Study Guide v2.62 – Aaron Balchunas
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Layer-3 Routing
Layer-3 routing is the process of forwarding a packet from one network to
another network, based on the Network-layer header. Routers build routing
tables to perform forwarding decisions, which contain the following:
• The destination network and subnet mask
• The next hop router to get to the destination network
• Routing metrics and Administrative Distance
Note that Layer-3 forwarding is based on the destination network, and not
the destination host. It is possible to have host routes, but this is less
common.
The routing table is concerned with two types of Layer-3 protocols:
• Routed protocols - assigns logical addressing to devices, and routes
packets between networks. Examples include IP and IPX.
• Routing protocols - dynamically builds the information in routing
tables. Examples include RIP, EIGRP, and OSPF.
Each individual interface on a router belongs to its own collision domain.
Thus, like switches, routers create more collision domains, which results in
fewer collisions.
Unlike Layer-2 switches, Layer-3 routers also separate broadcast domains.
As a rule, a router will never forward broadcasts from one network to
another network (unless, of course, you explicitly configure it to). ☺
Routers will not forward multicasts either, unless configured to participate in
a multicast tree. Multicast is covered in great detail in another guide.
Traditionally, a router was required to copy each individual packet to its
buffers, and perform a route-table lookup. Each packet consumed CPU
cycles as it was forwarded by the router, resulting in latency. Thus, routing
was generally considered slower than switching.
It is now possible for routers to cache network-layer flows in hardware,
greatly reducing latency. This has blurred the line between routing and
switching, from both a technological and marketing standpoint. Caching
network flows is covered in greater detail shortly.
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